Bioreactors used to treat wastewater produce a variety of gases as normal by-products of their operation. Typically, the primary gases produced by the microbial activity of a bioreactor are carbon dioxide and nitrogen. These gases are produced too rapidly in quantities too large to remain dissolved. Consequently, they form bubbles which become trapped in the filter media or packed bed filters of the bioreactor system.
As the quantity of gas trapped in the filter media or packed beds accumulates, water flow through the affected filters becomes increasingly restricted, and eventually adversely affects performance and efficiency of the overall wastewater treatment system. Therefore, it is necessary to periodically perform a degas procedure on the filter media or packed beds of the treatment system to remove trapped gas bubbles and solids from the filters in order to restore system performance and efficiency. It is desirable to accomplish this task as quickly as possible and with a minimum of disruption to the normal operation of the wastewater system.
The degassing of wastewater system filters is known in the art. A common degassing technique is to backwash or back-flush the filter media or packed bed with backwash water to dislodge and carry away the trapped gases.
Neff et al., U.S. Pat. No. 3,994,803 discloses an apparatus and method for denitrification of wastewater using a backwash reinnoculation technique. The invention comprises a container within the reactor for collecting a portion of the reactor water in a container and releasing the contained reactor water onto the filter bed when normal operation is resumed. Angelino, U.S. Pat. No. 5,288,398, discloses a backwash device and method which is automated for backwashing filter cells one at a time, wherein the backwash liquid when cleaning one cell is prevented from moving to a further cell until the operation is completed on the one cell. Savage, U.S. Pat. No. 4,159,945, discloses a method for denitrification of aqueous nitrate-containing solutions. The filter media comprises particles capable of entrapping bubbles of nitrogen gas generated by bacterial. The entrapped bubbles are removed by backwashing.
Although these techniques may be effective to different degrees in removing the trapped gases and restoring full system flow and filter capacity, they require at least the part of the wastewater system containing the filter to be degassed to be isolated in some manner from the rest of the treatment system during the degassing process. Typically, the degassing process is discretely accomplished, meaning that a single filter is isolated from the treatment system and the degassing process initiated and completed before a next filter is initiated into the process. Further, while a specific filter is removed from the system, remaining filters in the system have to handle the additional work load of the isolated element.
Savage et al., U.S. Pat. No. 3,970,555 disclose an apparatus and gas collection and venting system for removing gas bubbles from filter media. Gas bubble removal is accomplished or enhanced by an entrapped gas collecting means disposed within the filter bed and venting the collected gas bubbles to the atmosphere throughout a conduit means or venting pipe. Pressurized air or water is introduced into the lower portion of the filter bed to dislodge the gas bubbles from the filter media and drive them upward where they can be vented to the atmosphere. Tymoszezuk, U.S. Pat. No. 3,853,752 discloses a process of and apparatus for treating wastes by a combined activated sludge and biological filter that uses uplifting aeration for backwashing.
It would be desirable to have a degassing method available which accomplishes a full degassing of filters with the benefits of reducing the amount of time it takes to degas a specific filter or packed bed element, increasing the number of filters being degassed during a single degassing process and reducing wear and tear on system valves and equipment and not increasing the load on the other working filters of the treatment system while a selected filter is being degassed.